Modified glance worker to process the new exists notification
removed created_at from list of fields that are verified in image.delete verification
Added warning when invalid payload is encountered while saving exists notification
Ensured that image verified event is sent only once per owner for all his images
- Made changes so that we verify each group of exists messages with
  the same owner and raw_id separately. This ensures that if there are
  multiple exists notifications for each owner pending verification,
  we send an individual verified notification for each of them.

- Fixed race condition which was causing multiple verified messages to be
  sent by starting the async verify process per owner instead of per image.